Wireless links are widely used in modern communication and a multitude of wireless communication systems have been developed to provide such wireless communication. Well known wireless communication systems are e.g. the Global System for Mobile communications (GSM), the General Packet Radio Service (GPRS), the Universal Mobile Telecommunications System (UMTS) and other cellular technologies or similar. Other well known examples of wireless communication systems are the Wireless Local Access Networks (WLAN) of various types and the Worldwide Interoperability for Microwave Access (WiMAX).
The selection of a relevant transport format to be used for communicating via a wireless link is crucial to obtain an advantageous performance, e.g. such as a high throughput. Thus, most modern wireless communication systems are configured to dynamically select an advantageous transport format among a set of available transport formats for sending information to a receiver via a wireless link.
Generally, a transport format is the manner in which the information is conveyed over a wireless link. This may e.g. include the used modulation and/or coding and/or power level and/or frequency or number of transmission layers (MIMO rank) etc for constituting the wireless link.
The 3rd Generation Partnership Project (3GPP, see e.g. www.3gpp.org) has specified that a transport format should be chosen based on so-called Channel Quality Indicator reports (CQI-reports) in connection with the so-called Long Term Evolution (LTE), see e.g. the specification 3GPP TS 36.213 v8.6.0 “E-UTRA Physical Layer Procedures”. The CQI-reports are typically derived by the receiver to reflect channel quality and interference levels of the wireless link in question. The CQI-reports are then communicated back to the transmitter over a signaling channel of the wireless link. For example, the CQI-reports may be derived by a mobile terminal such as a User Equipment (UE) or similar and then sent back to a base station such as a Node B or similar. In downlink the received CQI-reports are then used by the transmitter to select the transport format that enables a transmission of as much user data as possible using as little resources as possible. However, for uplink there are typically no CQI-reports but the transport format selection is done in the base station directly on uplink measures such as Signal-to-Noise-Ratio (SNR) and the selected uplink transport format is then sent to the mobile terminal.
According to the specification 3GPP TS 36.213, V8.6.0 a UE or similar shall, based on an unrestricted observation interval in time and frequency, derive for each CQI value reported in uplink subframe n the highest CQI-index between 0 and 15 as defined in a table 7.2.3-1 of said specification. The table 7.2.3-1 is substantially identical to table 1A in FIG. 1 of the appended drawings, which defines 16 different CQI-indexes 0-15 corresponding to 16 different transport formats TF0-TF15. However, the derivation of a CQI-index must satisfy the condition that; a single Physical Downlink Shared Channel (PDSCH) transport block with a combination of modulation scheme and transport block size corresponding to the CQI-index, and occupying a group of downlink physical resource blocks termed the CQI reference resource, could be received with a transport block error probability (BLER) not exceeding 0.1. If this condition is not satisfied by CQI-index 1, then CQI index 0 shall be derived. The resulting BLER will then be less than 0.1 in an ideal case. However, the reported CQI will be delayed and degraded by other measurement errors. To mitigate that an outer-loop CQI-adjustment may e.g. be designed measuring BLER and adjusting with a margin so as for example to target an average Hybrid Automatic Repeat Request (HARQ) retransmission of 10%.
FIG. 2a illustrates schematic graphs of the throughput of the transport formats TF1-TF15 in table 1A as a function of Signal-to-Noise-Ratio (SNR). The graphs TF1-TF15 can e.g. be obtained by link simulations or similar. In addition, FIG. 2b is a schematic illustration of the throughput of one representative transport format TFi being valid mutatis mutandis for all transport formats TF1-TF15. As can be seen in FIG. 2b, the schematic throughput of the transport format TFi has a substantially skewed S-shape. The throughput is maximised above a certain SNR-high value and it is minimized (substantially zero) below a certain SNR-low value. The throughput increases at an increasing rate as the SNR rises above the SNR-low value until the SNR-value reaches a SNR-linear-low value, thus forming a lower knee. Above the SNR-linear-low value the throughput increases at a substantially linear rate until the SNR-value reaches the SNR-linear-high value, thus forming a substantially straight line. Above the SNR-linear-high value the throughput increases at a decreasing rate until the SNR-value reaches a SNR-high value, thus forming an upper knee.
It should be emphasised that the graphs in FIG. 2a are merely examples of throughput curves. The various available transport formats may be represented by several possible throughput curves to optimize against. For example, it is possible to only consider throughput curves without HARQ retransmissions. But it is also possible to take HARQ retransmission effects into account, where e.g. chase combining or incremental redundancy gains are taken into consideration.
In view of the specification 3GPP TS 36.213, V8.6.0 and table 1A in FIG. 1 comprising the transport formats 1-15 as schematically illustrated in FIG. 2a-2b it can be concluded that a UE or similar will select the transport format with the highest throughput at the current SNR-value, corresponding to a CQI value for the wireless channel in question, provided that the BLER for the transport block does not exceed 10%. Hence, at an excellent SNR-value transport format TF15 (CQI-index 15) will be used, which according to table 1A has a 64 QAM modulation with a code rate of 948×1024 bits/s (6 bits/symbol). If the SNR-value deteriorates such that the BLER exceeds 10% the next transport format TF14 (CQI-index 14) will be selected, which according to table 1A has a 64 QAM modulation with a code rate of 873×1024 bits/s (6 bits/symbol). If the SNR-value deteriorates further such that such that the BLER exceeds 10% again then the next transport format TF13 (CQI-index 13) will be selected, and so on until the first transport format TF1 (CQI-index 1) is selected, which according to table 1A has a QPSK modulation with a code rate of 78×1024 bits/s (2 bits/symbol). Lower SNR-values are out of range with respect to the transport format selection provided for according to the specification 3GPP TS 36.213, V8.6.0.
The transport formats 1-15 (modulation and coding combinations) according to FIG. 1 is only used for CQI reporting. The actual transport formats used at transmission can be a larger set than the reported 15 enabling a refined granularity. The selection of used transport formats is an eNodeB vendor specific choice. The eNodeB does not necessary (and typically not) follow the recommended transport formats indicated by CQI.
However, using a HARQ BLER target or BLER target as described above is not optimal for the whole range of radio link quality and reported CQI. According to this approach, a current transport format TFi+1 will be replaced with a new transport format TFi with a lower throughput when the BLER reaches 10% even if the current transport format would have provided a higher throughput at higher BLER values (i.e. BLER≧10%).
Hence, in view of the above there seems to be a need for improvements directed to the selection of a transport format to be used by a wireless link in a wireless communication system.